Every embryo of all animals go through the stage of gastrula. The most primitive animals like polypes are at this stage even as adults. The first multicelular organisms were at this stage too. So this is the stage which presume how our first ancestors looked like. So yes ontogeny recapitulate phylogeny. The next stages probably recapitulate phyllogeny specificaly in a particular evolutional branches for example our embryo have in the first stages tail like our ancestors did.
I dont know much about embryology but im eager for the course the next year.

There are several theories about how metazoas become. Heckel`s theory that first metazoas were Cnidaria and Spongia derived from colonial flagellata is one of two major theories. He said that ontogeny is recapitulation of filogeny based on the examination of embional stages of animals. But he said that embrional stages of higher animals are something like adult stages of lower animal from wich they evolved. That is obviously not correct. Nevertheless the ontogeny can be taken with much reserves as a guide line in determining the evolution line of some group of animals. If they have different embrional development they are not in the same evolutionary line. The other theory is made by Hadzi. It is called turbelaric theory. He said that first metazoas were like todays Turbelaria and that they derived from multinuclear Ciliophora. This look more plausible to me but neather theory explaines all. And there is 3rd group of theories wich says that different group of meatazoas derived from different groups of lower animals.

Heackel's theory was disprove? Why? When? About Hadzy theory, it is the same i think, cause animals which was evolving had to go through the same stages as are described in embryology and in Heackels theory, for example they had to be the mass of cells (like volvox or morula with the exception it had to be diploid) then create blastopore to focus digestion to the primary cavity and other processes (like gastrulation or more higher cephalization). So ontogeny must recapitulate phylogeny in both theory. [/quote]